CHLOROPLAST CYTOCHROME B(6) F AND ATP SYNTHASE COMPLEXES IN TOBACCO -TRANSFORMATION WITH ANTISENSE RNA AGAINST NUCLEAR-ENCODED TRANSCRIPTSFOR THE RIESKE FES AND ATP-DELTA POLYPEPTIDES/

Tobacco plants with reduced amounts and activities of both the chlorop last cytochrome b(6)/f and ATP synthase complexes have been produced u sing antisense RNA techniques. Antisense constructs were generated fro m tobacco cDNA clones coding for the Rieske FeS protein and the delta subunit of the b(6)/f and ATP synthase complexes respectively. Transfo rmants with altered activities were selected using pulse-modulated flu orescence measurements. The b(6)/f transformants showed high levels of steady-state fluorescence and reduced levels of both photochemical an d non-photochemical quenching. In striking contrast, the ATP synthase transformants showed low levels of steady-state fluorescence and great ly increased levels of non-photochemical quenching. Transformants with a range of suppression were isolated for both constructs, in some cas es with photosynthesis reduced to less than 10% of wild-type values. T he most severely affected transformants showed extremely slow growth a nd in some cases they were unable to grow and produce seed. Progeny fr om the R1 seed from several cytochrome b(6)/f transformants have been analysed and show segregation of phenotypes ranging from intermediate to severe in repression. Intermediate and severe phenotype plants show ed a reduction in Rieske FeS mRNA of more than 90% while FeS polypepti de was reduced to 60 and 86% of wild type. There was a strong correlat ion between photosynthesis at air and Rieske FeS polypeptide content i n the antisense plants suggesting that the cytochrome b(6)/f complex w as a major determinant of photosynthetic rate under these conditions. Photoinhibition studies of FeS antisense plants showed that there was a reduced activity of the xanthophyll cycle in the most severe plants, consistent with a reduction in the transthylakoid pH gradient and a l owered non-photochemical quenching. Preliminary studies of the ATP del ta antisense plants showed that they also had reduced levels of mRNA a nd ATP delta polypeptide.